Letter series type of questions usually consists of a series of small letters which follow a certain pattern. However, some letters are missing from the series. These missing letters are then given in a proper sequence as one of the alternatives. The candidate is requitred to choose this alternative as the answer.
c _ bba _ cab _ ac _ ab _ ac
The series is cabbac / cabbac / cabbac.
Thus, the pattern cabbac is repeated.